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When a large number of cables are laid in groups, will the current carrying capacity of the cables be reduced?
Release time:
2020-01-02
When a large number of cables are laid in groups, the current carrying capacity of the cables is reduced due to the mutual heating effect. Cables with large specifications sometimes need to be considered with two or more smaller parallel cables instead, because large cross-section cables will reduce the current carrying capacity per unit section due to skin effect and proximity effect.
When a large number of cables are laid in groups, the current carrying capacity of the cables is reduced due to the mutual heating effect. Cables with large specifications sometimes need to be considered with two or more smaller parallel cables instead, because large cross-section cables will reduce the current carrying capacity per unit section due to skin effect and proximity effect. On the other hand, the ratio of the surface area to the cross-sectional area of the large-section cable is reduced so that the large cable has poor heat dissipation capability. If multiple cables are used in parallel, the relative position of each cable should be considered to reduce the uneven distribution effect of cable current carrying capacity.
For cables laid in underground pipelines, when using load factors, the heat capacity of the average heat loss of the pipe group and the surrounding soil should be considered. The temperature of the underground part varies with the average heat loss, so a higher short-time load factor is allowed, which is the ratio of the average load to the peak load, usually measured on the basis of the average day and night load. The peak load generally refers to the average value of the maximum load during 0.5~1h that occurs within 24h. For directly buried cables, the average surface temperature can be limited to 0~60 ℃ according to soil conditions to prevent soil moisture loss and cable thermal breakdown.
When the cable is close to other loaded cables or heat sources, or when the ambient temperature exceeds the ambient temperature of the specified cable current carrying capacity, the rated current carrying capacity of the cable must be reduced. The normal ambient temperature of the cable installation is the temperature at which the cable is installed when the cable is not under load. In order to properly determine the cable specifications required for a given load, this temperature should be thoroughly understood. For example, the ambient temperature of a cable laid apart from other cables in the air refers to the temperature before the cable is loaded. For cables in the air, it is also assumed that there is enough space around the cables to dissipate the heat generated by the cables and will not increase the temperature of the entire room. If the above-mentioned correct conditions are specified, the following environmental conditions can be used to calculate the current-carrying capacity of the cable.
Urban high and medium voltage distribution lines shall use cable lines in the following cases. Urban prosperous areas, important sections, main roads, and areas with special requirements for urban planning and city appearance and environment; Serious corrosion areas that are technically difficult to solve; Sections of key scenic tourist areas; Important power supply sections of major coastal cities that are vulnerable to salt pollution or tropical storms; Other sections required for power grid structure and operation safety.
Urban low-voltage distribution lines should adopt urban central areas with high cable line load density under the following conditions: Newly-built residential quarters and high-rise building quarters with large construction area; According to the planning, it is not suitable to pass through the streets or areas of overhead lines and crowded areas of entry and exit lines; After technical and economic comparison, it is more appropriate to use power county group-level lines. When the cable line should be used but the underground does not have the conditions, the insulated cable overhead laying can be used.
For indoor low-voltage cables, the current flow meter on the national electrical regulations is based on an ambient temperature of 30°C. However, in the summer months in most areas, at least 40°C is appropriate for some parts of the building. The most unfavorable heat source near the cable must be considered when determining the current carrying capacity of the cable. Local overheating of the cable may be caused by the steam pipe or the heat source close to the cable, or it may be caused by the cable passing through the boiler room or other high temperature places. In order to avoid such problems, it may be necessary to change the line.
For outdoor cables installed in the shade, the maximum ambient temperature is generally 40°C, and for cables installed in the sun, the maximum ambient temperature is generally 50°C. In using these ambient temperatures, it is assumed that the maximum load occurs at exactly the specified ambient temperature. During the hottest hours of the day, or when the sun is at its worst, some circuits are not operating at full capacity. Under such conditions, the use of ambient temperature of 40 ° C for outdoor cable from the safety aspect, is more reasonable.
The ambient temperature for underground cables varies in different parts of a country. In northern China, the ambient temperature is usually 20 ℃, while in the central region, it is usually 25 ℃. For the southernmost and southwest ends, the ambient temperature may be 30 ℃. These geographical boundaries of ambient temperature cannot be precisely delineated. In determining the ampacity of a cable, the thermal properties of the medium surrounding the cable are important parameters. The type of soil in which the cable or cable pipe block is buried has a significant impact on the current carrying capacity of the cable. Porous loose soils, such as gravel and ash backfill, usually have a higher temperature and lower capacity than sand or clay.
Therefore, before calculating the cable specifications, you should know the type of soil and soil thermal resistivity. The water content of the soil also has an important influence on the current carrying capacity of the cable. In dry areas, in order to compensate for the increase in thermal resistance due to lack of moisture, the rated current carrying capacity of the cable must be reduced, or other precautions must be taken. On the other hand, in areas that are often wet underground or affected by tidal water, the cable can pass a larger current than normal.
When directly buried in permafrost areas, it should be buried below the permafrost layer. When it is impossible to bury deep, it can be buried in the dry permafrost layer with good soil drainage or backfill, and other measures can also be taken. Directly buried cables must not be located directly above or below underground pipelines. Separated by partition board to 0.25m; When passing through the pipe with cable, it can be 0.1m; Special circumstances can be reduced. When directly buried in non-permafrost areas, the cable burial depth shall not be less than 0.3m from the cable sheath to the foundation of underground structures.
When directly buried cables cross railways, highways or streets, they should wear protective pipes, and the protection scope is more than 0.5m beyond the roadbed, street pavement and drainage ditch edge. For cable lead-in structures directly buried, protective pipes shall be set at the through wall holes, and the pipe openings shall be blocked by water. For the joint configuration of directly buried cables, the clear distance between the joint and adjacent cables shall not be less than 0.25m. The joint positions of parallel cables should be staggered with a clear distance of not less than 0.5m. The joint placement at the slope terrain should be horizontal. For the cable joint of important circuit, the cable should be laid in the local section starting from about 100mm on both sides, and the cable should be laid in the way of reserve amount. When special soil replacement is adopted for backfilling of directly buried cables, the soil quality of backfilling shall be non-corrosive to the outer sheath of cables.
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